With the progress of practical application of optical communication systems by the development of optical fibers, the development of various optical communication devices using an optical waveguide structure has been demanded. In general, characteristics required for optical waveguide materials include low light propagation loss, possession of heat resistance and humidity resistance, and controllability of the refraction index and film thickness. With respect to these requirements, silica-based optical waveguides have hitherto been chiefly investigated.
However, in construction of optical fiber networks inclusive of WDM communication, it is essential to reduce costs for fabricating various devices. Accordingly, in order to apply polymer materials that can be mass-produced and subjected to large-area processing to optical waveguide materials, organic materials inclusive of polymethyl methacrylates, polycarbonates and polystyrenes have been investigated in recent years. However, in the case where such polymers are subjected to hybrid integration with a laser diode, a photo diode, etc., they have the defect that the range of their use is very limited, because their heat resistance in a solder reflow step is not sufficient. Of a number of polymer materials, polyimide resin-based materials have the highest heat resistance, so that they have recently attracted a great deal of attention as optical waveguide materials.
Optical circuits made of a polyamide resin have hitherto been generally formed by the following dry process. That is, a polyamic acid as a polyimide resin precursor is first dissolved in a polar solvent such as N,N-dimethylacetamide or N-methyl-2-pyrrolidone to prepare a polyamic acid varnish, which is applied onto a substrate by spin coating or casting and heated to remove the solvent and undergo ring closure of the polyamic acid for imidation, thereby forming a polyimide resin film, and then, a pattern is formed by reactive ion etching (RIE) using oxygen plasma etc.
However, according to the conventional dry process in which the polyimide resin film is subjected to reactive ion etching to form a pattern as described above, not only it takes a long period of time to form an optical circuit, but also the problem of reducing costs is not solved yet because a processing region is restricted. Further, according to such a dry process, a wall surface (side surface) of the pattern formed is not flat, so that scattering loss becomes large during wave guiding of light into the optical circuit.
Characteristics other than the loss required for the optical waveguide include good connection to optical functional parts and the capability of miniaturization. In the field of optical use such as optical communication, optical measurement or optical recording, a number of optical functional parts have been used for the purposes of switching, branching and connection of optical paths, and polarization, amplification, interference and diffraction of light waves, etc. As for these optical functional parts, respective parts to which functions are independently given are previously prepared, and then, these optical functional parts are combined to construct a desired system. Also in such a field of optical use, it is predicted similarly to the field of electronic use that developments will proceed toward high-density optical devices, highly integrated systems by three-dimensional lamination, miniaturized systems, etc. Accordingly, the development of techniques for unitizing or modularizing the optical functional parts has been demanded.
The optical functional parts are generally precision parts comprising materials such as inorganic glasses, metal oxides or plastic materials, so that it has been desired that actions by heat, pressure, reactive gas, etc. given in a unitizing step and a modularizing step are limited in terms of time and space in the greatest extent possible. Processing methods utilizing light are essentially suitable means for processing on any sites, and there is the possibility that optical processing can be easily conducted by utilizing polymer materials (plastic materials).
Specifically, the polymer materials have the feature that their thermal conductivity is low, so that it is liable to easily store heat. That is, in the polymer materials, the thermal motion thereof easily occurs compared to the inorganic glass materials, and only a small amount of heat is necessary for movement or reaction. There is therefore the possibility that an induced structure is formed even at relatively low irradiation energy, compared to the inorganic glass materials. Accordingly, the formation of the induced structure of the polymer materials using an ultra short pulse laser particularly has the advantage that it can be formed at any sites and in situ by irradiation of a lower-energy laser beam, compared to the inorganic glass materials.
On the other hand, with respect to miniaturization, so-called three-dimensional optical waveguides in which the optical waveguides are three-dimensionally fabricated have been known. As methods for producing the three-dimensional optical waveguides comprising polymer materials, a method using a gray mask or a shadow mask and a method according to a laser beam have hitherto been known (see, for example, Patent Document 1).
However, in the above-mentioned method using the gray mask or the shadow mask, not only it is necessary to form a clad material and a core material separately, but also RIE processing must be used. It has therefore the problem of low productivity. Further, the above-mentioned method according to the laser beam has the advantage that the process itself is simple and a core having a circular cross section can be formed. However, in order to modify the polymer itself, there is the restriction that an extremely high power laser must be used.
Patent Document 1: JP 2002-14246 A